1) Field of the Invention
The present invention relates to coating formulations useful for the formation of a back layer on heat-sensitive recording materials such as wax-type thermal transfer materials or sublimation-type thermal transfer materials.
The present invention is also concerned with heat-sensitive recording materials having a back layer coated therewith.
2) Description of the Related Art
Conventionally-known transfer processes include thermal ink-transfer processes and sublimation-type thermal transfer processes. According to the former processes, a heat-resistant recording layer is formed on a base sheet such as a polyester film by having a dye or pigment carried together with a binder resin on one side of base sheet. The base sheet is heated in a pattern from the back side thereof so that the dye or pigment is transferred onto a material to be transferred. According to the latter processes, a heat sub-limation dye is employed and the dye alone is caused to sublimate so that it is transferred onto a material to be transferred.
In each of these processes, thermal energy is applied from the back side of a base sheet. The back side of the base sheet of a heat-sensitive transfer material to be employed is therefore required to have sufficient lubricity, releasability, non-stickiness and the like so that a thermal head does not stick on the back side, in other words, does not develop any sticking problem.
It has therefore been the conventional practice to form a layer of a silicone resin, melamine resin, phenol resin, polyimide resin, epoxy resin or modified cellulose resin or a mixture thereof on the back side of the base sheet of a heat-sensitive recording material (see Japanese Patent Publication No. SHO 58-13359).
Most of these resins are, however, thermosetting resins making use of various curing agents. A heating step is therefore required for curing upon formation of a back layer on a heat-sensitive recording material, leading to the problem that the production process is cumbersome. Moreover, so-formed back layer itself is weak and is inferior in adhesion in spite of its high melting point, leading to the problem that the film layer may peel off as dust particles upon printing and may give a trouble to a thermal head.
With a view toward overcoming the problems described above, it has been proposed to add an isocyanate or the like to a thermoplastic resin such as an acrylic resin, polyurethane resin, polyester resin or polybutadiene resin and then to cure the resulting resin to provide a back layer and, further, to incorporate a lubricant or the like in the resulting resin to allow a thermal head to smoothly slide thereon (Japanese Patent Application Laid-Open No. SHO 59-225994).
According to the above patent publication, the curing of the isocyanate is said to proceed at room temperature. In practice, however, the above proposal has the problems that the pot life is insufficient and the resulting resin may not be cured sufficiently due to reactions of the isocyanate with water and other impurities.
In addition, the lubricant added to provide the back layer with smooth sliding property is accompanied by the potential problems that it may bleed out on the surface of the back layer if used for a long time, or may produce dusts on a head during printing.
The present inventors have already proposed that the use of a silicone copolymer resin in stead of the above-mentioned resins for the formation of a back layer can provide a heat-sensitive recording material excellent in various properties such as heat resistance, lubricity and non-stickiness (see Japanese Patent Application Laid-Open Nos. SHO 61-227087 and 62-202786).